This invention relates to an overcurrent detector for an inverter and, more particularly, to an improvement in the overcurrent detecting speed of an overcurrent detector for a voltage inverter, which voltage inverter is used in the drive control unit of a motor or the like, in driving an AC motor at variable speed.
Conventionally, an inverter is formed of a plurality of switching elements, to each of which elements a snubber circuit is connected in parallel. This snubber circuit is provided to suppress the maximum rise rate of the "off" potential dv/dt applied to the switching element, thereby preventing a malfunction of the switching element. The rate, "dv/dt", as used herein, is defined as follows:
"dv/dt--Minimum rise rate of the peak voltage, with respect to the time required to cause a switching from the off-state to the on-state."
When the dv/dt limit is exceeded, there is a potential danger that the switching element will be turned back on, when it should remain off. Since a snubber circuit includes a capacitor of relatively large capacity, a peak value of a charging current to this capacitor becomes fairly large, although for a short period. Therefore, the peak value of the current of the main circuit of the inverter to which the charging current for the snubber capacitor was added extremely exceeds the safety current value during the period of time when the snubber capacitor is being charged even when the stationary load current of the inverter is not larger than the safety current value. For example, assuming that the DC input voltage of the inverter is 600 V, snubber capacitor is 4 .mu.F, and series inductance of the filter capacitor of the main circuit is 10 .mu.H, the peak value of the snubber current is about 300 A and the current waveform width is about 20 .mu.sec. However, since the large current due to the charge of a snubber capacitor is not caused by abnormality or overload of an inverter, it is undesirable to make an overcurrent detector operative by means of the large current during the period of time when charging the snubber capacitor. Due to this, in a conventional overcurrent detector, a time lag over the snubber capacitor charging period of time would have been caused during the interval from the time when a switching element in the inverter has been switched to the time when an overcurrent is actually detected. Thus, even if an overcurrent actually flows due to, e.g., the commutation-failure (or shoot-through) of the switching element in the inverter, the detecting speed of this overcurrent will be slow with a conventional detector, resulting in low reliability of the protecting operation for the inverter or the load to be connected to the inverter. Particularly, when a gate turn-off thyristor (GTO) is used as a switching element, and an off-gate signal is applied to the GTO when an anode current exceeding its turn-off capability (overcurrent) is flowing, the GTO will be broken, since the dv/dt withstanding capacity is low. Therefore, in an inverter using a switching element having a low dv/dt withstanding capacity, such as a GTO, it is desired to realize the higher-speed overcurrent detecting operation than conventional one.